Electrical coils with permeable resin bonding permitting penetration of dielectric liquid therethrough



June 23, 964 F. s. NICHOLS ETAL 3,133,773

ELECTRICAL COILS WITH PERMEABLE RESIN BONDING PERMITTING PENETRATION OFDIELECTRIC LIQUID THERETHROUGH Filed Dec. 1, 1959 I (if?! 4 UnitedStates Patent g ELECTRICAL COILS WITH PERMEABLE RESIN BONDING PERMITTINGPENETRATION 0F DI- ELECTRIC LIQUID THERETlmOUGH Frank S. Nichols andEugene K. Steele, Pittslield, Mass, assignors to General ElectricCompany, a corporation of New York Filed Dec. 1, 1959, Ser. No. 856,4344 Claims. (Cl. 336-405) The present invention relates to electricalcoils and particularly to induction coils for electromagnetic apparatussuch as transformers and the like.

In the manufacture of coils of the above type, it has been the practiceto apply an insulating coating of varnish on the coil. By this means,the mechanical strength of the wound structure is increased foraffording better resistance to the stresses involved in the coilhandling and assembly procedures, and especially to the high mechanicalforces to which the coils may be subjected during operation of theelectrical apparatus. For example, under short circuit conditions, highmechanical stresses tend to separate the core windings both axially andradially and may lead to distortion or even rupture of the coilstructure. In the past, varnish compositions employed for coating thecoils have included alkyd resin, phenolic resin, and polyester resinvarnishes, the resin material being dissolved in a suitable volatilesolvent. However, it has been found that such varnish-treated coils arenot fully satisfactory for use in oil-filled apparatus where thedielectric oil or other liquid is intended to impregnate the coil toprovide adequate resistance to high electrical impulse conditions. Itappears that the previous varnish treatments resulted in aliquid-impermeable coating being formed over the coil structure whicheffectively prevented the ready penetration of the dielectric liquidinto the many small crevices and interstices inevitably present in thewound coil structure. As a result, there was a greater probability offailure due to impulse conditions than where such varnish treatment wasomitted. On the other hand, elimination of such varnish treatment couldbe made only at the sacrifice of short circuit strength of the coil.

It is an object of the invention to provide an electrical coil structurewhich has satisfactory mechanical strength to resist short circuitconditions as well as being capable of resisting high electrical impulseconditions.

It is a specific object of the invention to provide an electrical coilof the above type having an insulating coating thereon which affordssatisfactory mechanical bonding for the coil turns While permitting thedielectric liquid in which the coil is immersed to penetrate into theinterstices in the coil.

It is a further object of the invention to provide a method of treatingelectrical coils to impart the abovementioned improvements thereto.

Other objects and advantages will become apparent from the followingdescription and appended claims.

With the above objects in view, the present invention relates to anelectrical apparatus comprising, in combination, a. container having adielectric fluid therein, a coil in the container immersed in thedielectric fluid therein, the coil being formed by a wound conductorhaving an insulating coating thereon, the coil having intersticestherein and having deposited thereon a film formed of a 3,138,773Patented June 23, 1964 fused particulate resin material, the film beingdeposited on the coil from a suspension of polymer particles in anon-solvent liquid medium, whereby the deposited film firmly bonds theWound coil while permitting penetration of the dielectric fluid into theinterstices of the coil winding.

The invention will be better understood from the following descriptionand accompanying drawing in which:

FIG. 1 is a perspective view partly in section of an electricalinduction coil embodying the present invention; and,

FIG. 2 shows a transformer partly in section in which the FIG. 1 coil isincorporated.

Referring now to the drawing and particularly to FIG. 1 there is shown atypical transformer coil structure A comprising an inner insulatingcylinder 1 of pressboard or the like on which the coil turns are wound,the coil including a low voltage winding 2 and a high voltage winding 3which are radially separated from each other by duct forming spacers 4such as wooden strips or the like. Each coil winding consists of aplurality of radially superposed layers of helically wound wire 5, thewire being coated with insulating enamel such as a phenolicmodifiedpolyvinyl formal, and the layers of wire being separated from each otherby cellulosic paper insulation sheets 6. In accordance with theinvention, the surfaces of the coil structure are covered with a film ofresin 7 deposited from a suspension of polymeric particles in anon-solvent medium such as water.

FIG. 2 shows the coil structure of FIG. 1 operatively assembled in atransformer which comprises a tank 8 closed by a cover 9 and containinga dielectric liquid 10 such as mineral oil or the like. In thetransformer, the coil structure A encircles magnetic core 11, and thehigh and low voltage leads 12, 13 of the coil are respectively connectedto corresponding bushings 14, 15 mounted on cover 9.

By virtue of the particular type of resin coating provided on coil A, ithas been unexpectedly found that effective permeation of the dielectricliquid 10 into the inner structure and interstices of the coil unit isnot prevented and that as a resulta substantial decrease in the numberof failures on impulse testing of such coils is achieved. This benefitis obtained While still retaining a good bond between the coil turns bymeans of the resin coating, thus providing for adequate short circuitstrength of the coil.

The material which is applied to the coil to obtain the desired resincoating may be of known types of compositions consisting essentially ofsuspensions of extremely small particles of solid polymers in water, theparticle size being typically about 1 to 500 microns. These materialsare normally made by the emulsion polymerization of one or more liquidpolymerizable monomers, the emulsification of the original monomersproducing the very small size particles of polymer. The monomers usedmay be difunctional giving rise to thermosplastic polymers, commonexamples used commercially being vinyl acetate and various acrylicesters. Alternatively, the monomers may be polyfunctional provided thattwo of the functional sites may be preferentially reacted to produceminute particles of a thermoplastic polymer which by further reactioncan convert to a thermoset polymer. Materials of this latter typeprepared, for instance, from acrylonitrile, butylacrylate, methacrylicacid, and glycidyl methacrylate are described in Sanders Patent2,787,561.

A specific example of the latter composition is as follows:

Example 1 Ingredients: Parts by wt.

Polymerizable monomers- Acrylonitrile 63.0 Butyl acrylate 32.0Methacrylic acid 3.0 Glycidyl methacrylate 2.0 Polymerization initiatorSodium bisulfite 0.1 Potassium persulfate 0.3

Dispersing agentSodium lauryl sulfate c 0.5 Telogen-Dodecyl Inercaptan0.5 Water 200.0

Additional suitable compositions of thermosetting types are disclosed inthe patent to Erickson et a1. 2,580,901, of which the following areillustrative:

Example II Parts Styrene 180 Glycidyl methacrylate 20 Dioctyl sulfosuccinate 5 Water 580 Ammonium persulfate 0.1

Example III Parts Ethyl acrylate 120 Glycidyl methacrylate 80 Sodiumlauryl sulfate 1.5 Water 300 Ammonium persulfate 0.1

Other types of compositions of thermoplastic nature which may be used inpracticing the invention are:

Example IV Parts Water 512 Vinyl acetate 276 Ethyl acrylate 161 Butylacrylate 23 Sodium lauryl sulfate 7.5 Ammonium persulfate 0.5 Sodiumbicarbonate 0.4

Example V Parts Ethyl acrylate "29.26 Methyl methacrylate 14.41Methacrylic acid 0.67 Soduim lauryl sulfate 0.30 Sodium bicarbonate 0.05Ammonium persulfate (4% solution) 2.42 Sodium methabisulfite solution)1.00

Water 40.34 Ammonium hydroxide sufiicient to bring pH of productsuspension to 8.5 to 9.0.

In all cases, the product utilized in practicing the invention is astable suspension of minute fusible particles of polymer in anon-solvent medium, preferably water. In use, this suspension is coatedonto the coil and as it loses water by drying, the suspension becomesunstable and the polymer particles are deposited as a coating. Theparticles may subsequently be baked to fuse them together and, in thecase of the thermosetting types, to complete the second stage ofpolymerization and cure them. If desired, small amounts of solvent forthe polymer may be added to the suspension to facilitate fusion of thepolymer particles Without adversely affecting the operation of thematerial for the purpose of the invention. We have found that if anelectrical coil, particularly one insulated with cellulose material, isdipped in such a material and dried (or cured where appropriate), whilewater from the suspension may penetrate into the coil, the solidparticles are deposited on the surface only and that while such acoating provides eX- cellent mechanical reinforcement, it surprisinglydoes not prevent subsequent penetration of oil into the coil.

Several hundred distribution transformer coils have been treated with acommercially available acrylic polymer suspension of the above-describedtype, and it has been demonstrated that this treatment substantiallyimproved short circuit strength while satisfactory impulse tests onthese coils showed that complete penetration of the dielectric liquidtook place. In contrast, treatment of the coils with conventional alkydvarnish has been found in tests to result in six times as many failuresof the coils in impulse testing.

Another series of tests was conducted to determine the effectiveness ofthe described polymer suspension material in providing adequate bondingof the coil turns to resist high electrical and mechanical stresses. Inthese tests, in which the tested coils were subjected to short circuitconditions, it was found that coils having installation pretreated withshellac in accordance with a commonly employed procedure withstood only37 times the normal operating current before showing damage due tophysical shifting of the turns, whereas similar coils which had beenadditionally treated by dipping them into an aqueous acrylic polymersuspension of the above-described type withstood 50 times the normaloperating current before such damage resulted. This test thusdemonstrated the excellent coil-bonding properties of the poly mersuspension coating material.

In a typical process for treating transformer coils in accordance withthe invention, the coil is dipped at room temperature in a suspension ofpolymer material having the composition set forth in Example I above,the coil being typically formed of wire covered with a polyvinyl formalenamel and wound on a porous laminated paper or pressboard winding form.The viscosity of the suspension bath may be adjusted as desired byadding. water. The dipping period is not critical and may vary from afew seconds to several minutes. It has been found that due to the natureof the polymeric suspension even a single short dipping period iseffective to provide adequate bonding of the coil turns to provide highresistance to short circuit conditions, in contrast to conventionalvarnishes which normally require a relatively long or repeated dippingto provide equivalent mechanical strength. After the coil is dipped, itis removed from the bath and allowed to drain for a short period. Thereafter, the coil is allowed either to air dry and then curing is effectedduring the usual transformer drying cycle,

cure the polymer.

The bake cycle, as indicated above, is used either to fuse the polymericparticles (as in the case with suspensions of thermoplastic materials,e.g., polyvinyl acetate) or to fuse and cross-link the polymer chain (asin the case of thermosetting materials such as in the above-mentionedSanders patent). Thereafter, the dried core and coil assembly is placedin a transformer tank and the tank is filled with appropriate insulatingliquid.

While it appears that improved results are obtained by the presentinvention because the dielectric liquid of the electrical apparatuspermeates the coated coil more readily than in the case of prior typesof coated coils, the reason for the improved permeability is not fullyunderstood. It may be that the particulate resin material, althoughfused to a practically continuous film, leaves unbridged gaps especiallyat the ends of the coil between layers, through which the dielectric oilmay pass, whereas prior solution-types of resin coatings have formed animpervious bridge across such irregular areas or fissures and thedielectric liquid is thus elfectively prevented from entering theinterior of the coil to saturate the interstices therein.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electrical apparatus comprising, in combination, a containerhaving dielectric fluid therein, a coil in said container formed by awound conductor having an insulating coating thereon, said coil havinginterstices therein and having deposited only on the outer surfacesthereof a film composed of a fused particulate resin material, saiddeposited particulate film being porous and permeable to said dielectricfluid by both radial and axial penetration through the wound coil,whereby the deposited film firmly bonds the wound coil while permittingpenetration of the dielectric fluid into the interstices of the coilwinding.

2. An electrical apparatus comprising, in combination, a containerhaving a dielectric liquid therein, a coil immersed in said dielectricliquid in said container, said coil being formed by a wound conductorhaving an insulating coating thereon, said coil having intersticestherein and having deposited only on the outer surfaces thereof a filmcomposed of a fused particulate polymer material, said depositedparticulate film being porous and permeable to said dielectric liquid byboth radial and axial penetration through the wound coil, whereby thedeposited film firmly bonds the wound coil while permitting penetrationof said dielectric liquid into the interstices of the coil winding.

3. A coil structure permeable to dielectric liquids comprising, incombination, an elongated conductor wound into a plurality of turns,said turns being arranged in superposed layers, insulating materialsseparating said turns from each other, and a resin coating composed offused particulate polymer material overlying only the surfaces of saidcoil for bonding the winding turns together, said particulate resincoating being porous and permeable to dielectric liquid by both radialand axial penetration through the wound coil for impregnation of theinterstices of the wound coil by the dielectric liquid.

4. A coil structure permeable to dielectric liquids comprising, incombination, an elongated conductor wound into a plurality of turns,said turns being arranged in superposed layers, insulating materialseparating said turns from each other, cellulosic insulating sheetmaterial separating said layers from each other, and a resin coatingcomposed of fused particulate polymer material overlying only thesurfaces of said coil for bonding the winding turns together, saidparticulate resin coating being porous and permeable to dielectricliquid by both radial and axial penetration through the wound coil forimpregnation of the interstices of the wound coil by the dielectricliquid.

References Cited in the file of this patent UNITED STATES PATENTS2,246,159 Work June 17, 1941 2,814,581 Flynn Nov. 26, 1957 2,867,032Gehrke Jan. 6, 1959 2,942,217 Ford June 21, 1960

1. AN ELECTRICAL APPARATUS COMPRISING, IN COMBINATION, A CONTAINERHAVING DIELECTRIC FLUID THEREIN, A COIL IN SAID CONTAINER FORMED BY AWOUND CONDUCTOR HAVING AN INSULATING COATING THEREON, SAID COIL HAVINGINTERSTICES THEREIN AND HAVING DEPOSITED ONLY ON THE OUTER SURFACESTHEREOF A FILM COMPOSED OF A FUSED PARTICULATE RESIN MATERIAL, SAIDDEPOSITED PARTICULATE FILM BEING POROUS AND PERMEABLE TO SAID DIELECTRICFLUID BY BOTH RADIAL AND AXIAL PENETRATION THROUGH THE WOUND COIL,WHEREBY THE DEPOSITED FILM